Vanadium-base alloy having a high creep-rupture strength and an improved resistance to corrosion

ABSTRACT

AN ALLOY CONTAINING TRACES TO 5% TITANIUM, TRACES TO 1.2% ZIRCONIUM, TRACES TO 1.2% HAFNIUM, SAID ELEMENTS TITANIUM, ZIRCONIUM, AND HAFNIUM TOTALING AT LEAST O.1%, A TOTAL OF 5-20% OF THE ELEMENTS CHROMIUM AND MOLYBDENUM, TRACES TO 2% SILICON, TRACES TO 4% GERMANIUM, BALANCE VANADIUM, TRACES OF USUAL METALLIC IMPURITIES WHICH ARE DUE TO THE PROCESS OF PRODUCING THE VANADIUM AND, BASED ON THE VANADIUM CONTENT, 400-3000 P.P.,. OXYGEN, 100-1500 P.P.M. NITROGEN, AND 100-1500 P.P.M. CARBON, THE TOTAL OF OXYGEN, NITROGEN, AND CARBON BEING NOT IN EXCESS OF 4000 P.P.M.

United States Patent US. Cl. 75-134 V 8 Claims ABSTRACT OF THE DISCLOSURE An alloy containing traces to titanium, traces to 1.2% zirconium, traces to 1.2% hafnium, said elements titanium, zirconium, and hafnium totaling at least 0.1%, a total of 5-20% of the elements chromium and molybdenum, traces to 2% silicon, traces to 4% germanium, balance vanadium, traces of usual metallic impurities which are due to the process of producing the vanadium and, based on the vanadium content, 400-3000 p.p.m. oxygen, 100-1500 p.p.m. nitrogen, and 100-1500 p.p.m. carbon, the total of oxygen, nitrogen, and carbon being not in excess of 4000 p.p.m.

The present invention relates to a vanadium-base alloy which contains chromium and or molybdenum and has low contents of titanium, zirconium or hafnium. Shaped parts made of the vanadium-base alloy according to the invention have a high creep-rupture strength at elevated temperatures and an excellent resistance to corrosion by flowing molten sodium. The alloys according to the invention are particularly suitable for use in nuclear reactors as a structural material or for cladding fuel rods.

Due to their favorable neutron-physical properties, desirable behavior under irradiation, and high strength at elevated temperatures, vanadium-base alloys have gained special significance in the construction of nuclear reactors. Binary vanadium-base alloys are known which contain 5% titanium or 2.8% titanium (Journal of the Less Common Metals, vol. 12, 1967, pages 280-93). The vanadiumbase alloy containing 5% titanium has shown good creeprupture properties in tests carried out for as much as 1000 hours. Other binary vanadium alloys are known which contain 05-20% chromium or molybdenum (French patent specifications Nos. 1,473,176 and 1,473,177), as well as binary vanadium alloys containing 0.2-3% silicon (French patent specification 1,473,178).

Such alloys have been proposed as structural material for sodium-cooled nuclear reactors. These binary alloys which contain chromium and molybdenum can be shaped only if they do not contain more than 500 p.p.m. oxygen, 300 p.p.m. nitrogen and 300 p.p.m. carbon. The small contents of said elements result in a low mechanical strength, particularly at elevated temperatures. Binary vanadium-chromium alloys and vanadium-molybdenum alloys have very coarse grains. Finally, ternary vanadium alloys are known which contain 1.2-5.3% zirconium and 6-20.5% chromium or molybdenum and have been proposed for use in nuclear reactors (USAEC Report WCAP- 3487-15; 1966).

It is an object of the invention to provide a vanadiumbase alloy which has a good creep-rupture strength in prolonged tests and a high resistance to corrosion and which is particularly suitable in nuclear reactors as a structural material or for cladding tubes and has a creep- 3,695,866 Patented Oct. 3, 1972 "Ice rupture strength which is suitable for such use during a time of more than 10,000 hours.

To accomplish that object, the invention provides a substantially ternary vanadium-base alloy. The alloy according to the invention consists of a vanadium-base alloy containing at least one of the alloying elements titanium, zirconium or hafnium in an amount of 0.1-5 titanium, up to 1.2% zirconium, up to 1.2% hafnium, moreover, 5-20% chromium and/or molybdenum, balance vanadium in conjunction with 400-3000 p.p.m. oxygen, 1500 p.p.m. nitrogen, and 100-1500 p.p.m. carbon, the total of oxygen, nitrogen and carbon being not in excess of 4000 p.p.m., and with small amounts of usual metallic impurities, such as nickel, iron, chromium and copper, which are due to the manufacturing process.

The term usual metallic impurities relates to such metals which are contained in the vanadium as a result of the manufacturing conditions. Each of these impurities is present in an amount not in excess of 1000 p.p.m., e.g., up to 700 p.p.m. of each of the elements iron, chromium and nickel and up to 300 p.p.m. copper.

The addition of the elements chromium and/or molybdenum improves, above all, the resistance of the alloys to corrosion, e.g., by molten sodium at 600-800 C.

In addition to titanium or instead of titanium, the alloys according to the invention may contain the elements zirconium and hafnium, individually or in combination, in the composition range according to the invention. Zirconium and/or hafnium in amounts of 0.1-1.2% may be used alone or in addition to titanium and will increase the resistance to oxidation. Where improvement in ductility is desired it will also be desirable to replace the titanium content of the alloy according to the invention entirely or partly by zirconium and/or hafnium. Vanadium-base alloys which contain zirconium and/ or hafnium have, e.g., the composition:

0.8-1.2% hafnium and/or zirconium 8-12% chromium and/or molybdenum 400-800 p.p.m. oxygen 200-600 p.p.m. carbon 200-600 p.p.m. nitrogen, balance vanadium with impurities consisting of about 300 p.p.m. of each of the elements iron and nickel and about 100 p.p.m. copper.

The vanadium-base alloys in the composition range according to the invention contain as non-metallic substances the elements carbon and nitrogen in amounts of 100-1500 p.p.m. each and oxygen in an amount of 400- 3000 p.p.m. and the total of these elements does not exceed 4000 p.p.m. These elements amount preferably to 200- 600 p.p.m. carbon, 200-600 p.p.m. nitrogen and 400- 1500 p.p.m. oxygen. The inherent embrittling action of these elements on the vanadium-base alloy is largely eliminated by the additions of titanium, zirconium and/ or hafnium. The hard phases which are thus formed result also in a grain refinement and in an improved creeprupture strength.

An alloy which has shown particulary desirable creeprupture properties in prolonged tests under loads of 20-30 -kg./sq mm. and a high resistance to corrosion by flowing sodium at 600-800 C. has the following composition:

0.5-1.0% zirconium 14-16% chromium balance vanadium containing 200-600 p.p.m. carbon, 200-600 p.p.m. nitrogen, 400-800 p.p.m. oxygen and small metallic impurities consisting of about 300 p.p.m. iron, about 300 p.p.m. nickel and about 100 p.p.m. copper.

In the vanadium alloys containing 14-16% chromium and O.S-1% zirconium, the high chromium content results in a particularly high resistance to corrosion in flowing sodium at GOO-800 C. whereas the zirconium content which is lower than in previously known alloys results in a higher creep-rupture strength. As a result, an alloy in the composition range according to the invention containing, e.g., 0.5% Zr and 5% Cr has almost the same creep-rupture strength as one which has a high chromium content although it does not have the resistance of the latter to corrosion.

The fact that the contents of the elements titanium, zirconium and hafnium is lower than in the previously known alloys surprisingly results in a higher creep-rupture strength, as is apparent from the following table:

Life in hours at 650 C.

Alloy in the composition Known range alloy according to Stress .7% Zr, the invention, kg./sq. mm. 6% r) 0.5% Zr, Cr

WEIGHT CHANGE WITHIN 500 HOURS DUE TO CORROSION BY FLOWING SODIUM Weight change in mg./sq. cm.

Testing temperature 500 0. 550 0. 600 0.

Alloy (nominal composition):

Known alloy: V, 3% Ti, 15% Nb... 1. 2 0. 8 -0. 6 Alloys according to the invention:

V, 3% Ti, 15% Mo..- +0. 19 +0. 15 +0. 37 V, 3% Ti, 16% Or. 0. 16 -0. 02 +0. 03

The capture cross-section for fast neutrons is also lower than in alloys which contain columbium. I

The vanadium-base alloys according to the invention may be made by known metallurgical processes, e.g., in that the components of the alloy are fused together under a vacuum or in a rare gas atmosphere or by powdermetallurgical sintering processes. Suitable melting furnaces include, e.g., electron beam furnaces or are furnaces. Known processes, including extruding, forging, rolling and drawing, may be used to manufacture shaped parts from the alloys according to the invention.

The vanadium-base alloys in the composition range according to the invention have advantages. The surprisingly high creep-rupture strength, the small neutron capture cross-section, the low embrittlement which can be caused by an irradiation with neutrons at temperatures of 600-800 C., the high resistance to corrosion by flowing liquid alkali metals and the high deformability of the alloys are decisive requirements for the use of such alloys as a material for structural elements and as a cladding material for fuel elements in nuclear reactors.

The vanadium-base alloys in the composition range according to the invention can be used to special advantage as materials for structural elements which require a high creep-rupture strength at temperatures between 500 and 1000 C., preferably between 600 and 800 C., as well as a high resistance to corrosion by flowing liquid alkali metals, particularly sodium, a small tendency to become embrittled by the irradiation with neutrons at temperatures between 600 and 800 C. and a small neutron absorption, or any individual one of these properties. Finally, the alloys in the composition range according to the invention can conveniently be formed at elevated temperatures because they have a lower resistance to deformation than otherwise similar alloys which contain columbium.

Alloys in the composition range according to the present invention are used particularly as a material to make structural elements and as a cladding material for fuel elements in nuclear reactors, particularly in sodiumcooled breeder reactors.

The high resistance of the alloy according to the invention to corrosion enables the use of this alloy in chemical engineering. Owing to its high strength at elevated temperatures in conjunction with its relatively low density, the alloy is interesting as a material for aircraft and spacecraft.

What is claimed is:

1. A vanadium-base alloy consisting of traces to 5% titanium,

traces to 1.2% zirconium,

traces to 1.2% hafnium,

said elements titanium, zirconium and hafnium totaling at least 0.1% a total of 5-20% of at least one element selected from the group consisting of chromium and molybdenum. traces to 2% silicon,

traces to 4% germanium balance vanadium, wherein said vanadium contains traces of usual metallic impurities which are due to the process of producing the vanadium and, based on the vanadium content, 400-3000 p.p.m. oxygen, 100-1500 p.p.m. nitrogen, and 100-1500 p.p.m. carbon, the total of oxygen, nitrogen, and carbon being up to 4000 p.p.m.

2. An alloy as set forth in claim 1, which contains, based on its vanadium content, 400-1500 p.p.m. oxygen, 200-600 p.p.m. nitrogen, and 200-600 p.p.m. carbon.

3. An alloy as set forth in claim 1, in which said metallic impurities comprise at least one of the elements nickel, iron, chromium, and copper.

4. An alloy as set forth in claim 1, which contains at least 0.1% silicon.

5. An alloy as set forth in claim 1, which contains 0.5-1.5% silicon.

6. An alloy as set forth in claim 1, which contains at least 0.5% germanium.

7. An alloy as set forth in claim 1, which contains 1-3% germanium.

8. An alloy as set forth in claim 1, which contains at least 0.1% silicon and at least 0.5% germanium.

References Cited Rostoker: The Metallurgy of Vanadium, New York, Wiley (1958), pp. 86, 87, and relied on.

L. DEWAYNE RUTLEDGE, Primary Examiner E. L. WELISE, Assistant Examiner 

